TeX Live package installation

You may have gotten TeX Live in one of two fundamentally different
ways. The way to install and update packages depends on which way you
got your installation.

TeX provided by your operating system

First, you may have gotten a TeX installation that was based on TeX
Live but was packaged for your operating system. For instance, the free GNU/Linux
distributions, and the distributions from Red Hat, Debian, Ubuntu,
SUSE, etc., all make TeX installations derived from TeX Live available
through their normal packaging system (rpm, apt-get,
yum, etc.). If this is how you got your TeX, the timing and
content of updates is entirely up to your operating system
provider—contact them with any and all questions.

By the way, a native TL, which is typically installed under
/usr/local, and a TeX from your operating system can happily
coexist, each with their own completely independent trees and programs.
(Do not try to merge them!) So you can install a native TeX Live if
your vendor is not keeping up.

Native TeX Live

The other case is that you installed the “native” TeX
Live, e.g., from the DVD or over the net. Here, the first thing
to try, both to install new packages and to update already installed
ones, is to run (on a command line/system prompt) the command
tlmgr update --all (tlmgr is the command name of the
TeX Live package manager). If you would like to see what would be done
before doing it, run tlmgr update --list.

If that doesn't get the material that you want, then here is one
possibility. When you installed your native version of TeX Live, the
default was to include everything available for your platform. If you
accepted this default, called scheme-full, then the above
command gets you the freshest version of everything available. However,
if you chose a smaller scheme than the default, then you will get only
what is included in that scheme. Likewise, if you have manually removed
collections or packages (terminology
explanation), then those packages will not be automatically updated.
In these cases, to manually install or reinstall a package, run
tlmgr install pkgname.

If you accepted scheme-full, and you still didn't get the
material you were expecting, here are two other possibilities:
1) the TeX Live developers have not yet made the update.
This is usually done within a day or two of the upload to CTAN, but
occasionally it can take longer.
2) the package may not be available under a free software license,
among other issues, in which case it cannot be included in TeX Live (the package contribution page explains).

TeX Live terminology

To explain the terms schemes, collections, and packages used above:
the general idea of native TL installation is to choose one of the
available top-level schemes, each of which is defined as a set of
collections and packages. A collection is a set of
packages, and a package is what contains actual files. Each package
is in exactly one collection, no more and no less.